Neutrophil-specific antigens NA1 and NA2 are located on Fc-gamma-receptor IIIb (FcgRIIIb). The NA1 and NA2 forms of FcgRIIIb differ by four amino acids and their FcgRIIIB genes by five base pairs. The FcgRIIIB genotypes of 232 people were analyzed at four NA polymorphic sites using polymerase chain reaction and restriction enzymes. Two primer pairs were used to amplify genomic DNA. One amplicon was digested in NA1-positive people at base 227 by XmnI and in NA2-positive at 141 by Cac8I. The second was digested in NA2-positve people at two sites, base 147 by HindIII and base 277 by AclI or Bsp1406I.The NA1 gene frequency in 100 Japanese (0.66) was greater than in 53 African Americans (0.40) and 79 Caucasians (0.32). The number of people whose genotype was not clearly NA1 or NA2 was unexpectedly high'16 (7 percent). The frequency of these unusual genotypes was greater in African Americans (19 percent) than Japanese (4 percent) or Caucasians (2.5 percent). Analysis of FcgRIIIB at bases 141 and 227 identified four African Americans with one gene that was identical to NA1 at 227 and NA2 at 141. Further analysis of FcgRIIIB identified six African Americans who had a NA2 gene that lacked an NA2 restriction digest site at 277. Two Japanese and one Caucasian had an NA2 gene without an NA2 restriction site at 141. One Japanese had an NA1 gene with an NA2 restriction site at 141. One Japanese and one Caucasian had an NA2 gene without an NA2 restriction site at 147. The FcgRIIIB genes in two of these people have been sequenced and two new NA alleles have been identified. The NA1 gene is more prevalent in Japanese. Variations in the NA1 and NA2 alleles of FcgRIIIB exist and are more common in African Americans.